hw/char/serial: Remove unused prog_if compat property

hw/char/serial: Remove unused prog_if compat property

This property was added to preserve previous value when this was fixed
in version 2.1 but the last machine using it was already removed when
adding diva-gsp leaving this property unused and unnecessary.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Helge Deller <deller@gmx.de>
Link: https://lore.kernel.org/r/20250502095524.DE1F355D264@zero.eik.bme.hu
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>